Frozen soil is a type of soil medium which is rich in underground ice, very sensitive to temperature, and has rheological characteristics.
Qinghai-Tibet highway is the world's first asphalt road paved in a high-and-cold frozen soil area. The “black surface close over” characteristic of asphalt pavement makes the road absorb a mass of the sun's heat energy in warm seasons, and meanwhile, retards evaporation of water. But in cold seasons, massive heat accumulated in the subgrade can not be released. Since the frozen soil is very sensitive to temperature, repeated freezing and melting alternations make the subgrade instable, which results in frequent highway disease.
In plateau permafrost areas, descending of permafrost table under subgrade is the main cause of uneven settlement of subgrade, and threatens the stability of subgrade. Highway engineering in each cold region of the world are faced with similar problems. In order to curb the descending of permafrost table, subgrade protection measures are generally taken in frozen soil-related engineering. The protection measures mainly include: techniques such as subgrade lifting, ventilation tube, block stone (crushed stone) subgrade, block stone (crushed stone) revetment, thermal insulation layer, heat-conducting tube, diatomite revetment, sun shield, and so on.
The subgrade lifting and ventilation tube techniques are among the most commonly-adopted engineering measures in frozen soil protection engineering. In the 1985-1999 stage 3 of renovation and reconstruction of asphalt pavement in permafrost areas by Qinghai-Tibet highway research group, the subgrade in general was raised by approximately 3 meters. But after that, subgrade defects resulted from temperature differences between temperature fields on the sunny and shady sides began emerging. The left side (the sunny side) of the highway suffers from road border collapsing and longitudinal cracking; meanwhile the raised subgrade makes the thaw layer under the subgrade thickened, and settlement deformation of the subgrade becomes larger, which brings about massive hidden troubles for stability of the subgrade. Measures of pre-laying ventilation tubes in subgrade and embankment can also achieve some effect. However, since the ventilation tube measure is based on forced convection heat transfer mechanism, the ventilation tubes are effective only when they are laid a certain distance higher than the natural surface, which enlarges the side slope area of the embankment and results in shady-sunny effect of the subgrade. For this reason, the ventilation tube measure is not very effective in practical usage. Current various improved ventilation tubes proposed in subgrade of domestic plateau permafrost areas have similar effects.
The block stone (crushed stone) subgrade technique is also a protection measure which is for cooling the subgrade. The block stone (crushed stone) subgrade is ventilated and cooled by utilizing the pores in the block stone pile. Research results of LAI, Yuanming (“Temperature Features of Broken Rock Mass Embankment in the Qinghai-Tibetan Railway”, Journal of Glaciology and Geocryology, Issue 03, 2003) show that the ripped-rock subgrade cooling technique is related with temperature differences between the top and bottom of the ripped-rock layer and the riprap grain size. When a critical condition is hard to be met, the cooling property of the ripped-rock layer will not work. Since the ripped-rock layer is wide (generally 20 m in width), the middle part has poor or no ventilating property, which reduces the ventilating ability of the ripped-rock layer. In plateau permafrost areas, the temperature of the permafrost under the road does not go down, and even goes up in some areas. “Research Report on Heat Insulation Testing Project of Ripped-rock and Ripped-rock revetment, 2003: Sub-report of Report of Research results on key techniques about road bridge and culvert in thick subsurface ice regions (Bailu River)” by SUN, Zhizhong et. al. studies the cooling effect of block stone (crushed stone) revetment subgrade in Bailu River testing phase of Qinghai-Tibet railway, and points out that: the block stone (crushed stone) revetment subgrade has good cooling effect of the frozen soil at the foot of the slope, but the temperature of soil mass at the middle part of the subgrade is still relatively high, and the unevenness of temperature fields may results in uneven deformation of the subgrade and even longitudinal cracking, which may harm traffic safety.
In addition, industrial thermal insulation materials are also not applicable in high-temperature permafrost areas, while the industrial thermal insulation materials used in low-temperature permafrost areas may cause heat to accumulate in the subgrade which causes the permafrost table to descend.
Heat-conducting tubes have a problem of caloric leakage. Once the caloric leaks, the heat-conducting tubes will no longer be used. Further, the performances of the heat-conducting tubes depend on influences of climate conditions, unit weight and water content of surrounding soil mass.
Although diatomite revetment technique is a theoretically feasible measure for protecting permafrost, it is hard for diatomite revetment to function properly as a thermal diode in plateau permafrost areas due to insufficient water supply.
The sun shield technique has remarkable cooling function, is an effective engineering measure for lowering the temperature of the permafrost under the subgrade. But the sun shield technique has drawbacks such as high costs in building and repairing, vulnerability to harsh environment, e.g., gales, etc. of Qinghai-Tibetan Plateau, preventing the slope from slope vegetation, dangerous to drivers due to the light reflected by the white color of sun shield.
In view of the above current subgrade protection measures for highway in frozen soil areas, there is still no effective and low-cost protection measure.